1. Field of the Invention
The present invention relates to biometric data (e.g., fingerprint) sensing circuitry, and more particularly to a circuit and method for improving dynamic range in such circuitry.
2. Description of the Prior Art
Capacitive biometric data sensing circuits (also referred to as pixels) are well known. Such a circuit is shown and described in U.S. Pat. No. 6,512,381, incorporated herein by reference. More specifically, with reference to FIG. 1 hereof, there is shown a schematic illustration of a circuit for 10 biometric data sensing. FIG. 2 is an illustration of a device 20 implementing the circuit 10 of FIG. 1.
A common application of such circuits is imaging of a user's fingerprint pattern found on the tip of one of the user's fingers, for example for user identity verification. Such circuits sense field changes due in part to proximity of the finger of the user. These circuits are very sensitive, and are in fact able to detect, for example, differences in field strength in the presence of a peak of a fingerprint and field strength in the presence of a valley of a fingerprint on a pixel-by-pixel basis. For the circuit 10 of FIG. 1, in terms of the sensing capacitance, Csense, at capacitor 12,
      C    sense    =      {                                                      C                              S                ⁢                                                                  ⁢                0                                      ⁢                                                  ⁢            in            ⁢                                                  ⁢            absence            ⁢                                                  ⁢            of            ⁢                                                  ⁢            a            ⁢                                                  ⁢            fingerprint            ⁢                                                  ⁢            ridge                                                                                          C                                  S                  ⁢                                                                          ⁢                  0                                            ⁡                              (                                  1                  -                  α                                )                                      ⁢                                                  ⁢            in            ⁢                                                  ⁢            presence            ⁢                                                  ⁢            of            ⁢                                                  ⁢            a            ⁢                                                  ⁢            fingerprint            ⁢                                                  ⁢            ridge            ⁢                                                  ⁢                          (                              α                ⁢                                                                  ⁢                                  typ                  .                                                                          ⁢                  approx                  .                                                                          ⁢                  0.1                                            )                                          In terms of the voltages at the input node 14 and output node 16 of the circuit,
                                                    Δ            ⁢                                                  ⁢                          V              out                                                =                                                        Δ              ⁢                                                          ⁢                              V                in                                                          ·                                    C              in                                      C              sense                                                          (        1        )            Thus, the difference in output voltages between pixels can be used to generate a local image of the user's fingerprint.
The output of a typical sensing circuit is comprised of a “differential mode” (that portion of the output signal of interest, commonly referred to as data), a “common mode” (base data present in each pixel regardless of the presence of a finger), and noise, attributable to many source such as the design of the circuit itself, the environment in which the circuit is operated, etc. FIG. 3 illustrates a graph of the output voltage from a pixel as a ridge of a fingerprint passes over the pixel, showing differential and common modes (noise is omitted for clarity), for two respective pixels. A description of the timing of the various signals is beyond the scope of the present disclosure. If at the pixel location p1 a valley of a fingerprint is present over the sensing circuit, the output voltage from the pixel is V1. If at the pixel location p2 a ridge of a fingerprint is present, the output voltage is V2. It is the difference between V1 and V2 (ΔV) that is of interest. Thus, at the limit V1 can be considered the “common mode” (although in certain embodiments, a voltage measured in the absence of a finger may be considered the “common mode.”)
At present, the common mode may account for as much as 90% of the pixel output signal, with a mere 10% of that output signal representing data (the information needed to construct a biometric image such as a fingerprint). As the ratio of data (signal) to noise decreases in an output, the difficulty in accurately determining data in that output increases. It is a goal of circuit design to minimize the common mode so that the data is more easily and accurately recognized.
Commonly in the prior art, gain and offset adjustments are made to compensate in part for the common mode signal. However, adjusting the gain and offset affect not only the undesired “common mode” signal, but also the desired data (signal). Thus in the prior art, the output dynamics of the sensing circuit are not used efficiently to reflect the presence of surface modulations, i.e., the fingerprint features.
Compounding this poor signal-to-noise problem is a decrease in typical operating voltage for fingerprint sensors (driven, for example, by the desire to increase battery life in portable devices using such fingerprint sensors). Circuits of the type illustrated in FIG. 1 have typically been operated at 5 volts. This has provided acceptable dynamic range for sensing. Dynamic range is the greatest possible range of output signals, taking away the noise or common mode. Basically, dynamic range is the difference between an output signal in the presence of a ridge and in the presence of a valley. However, there is a trend to design biometric sensing circuits to operate at 3 volts or lower. In doing so, the dynamic range is compressed. That is, the actual voltage representing the data decreases to a point that it is difficult to detect, especially as compared to the voltage representing the common mode. Thus, as dynamic range is compressed, the noise portion of the pixel output dominates the data, and generation of a biometric image (e.g., fingerprint) becomes more difficult and less accurate (if possible at all).